Imaging of appendiceal mucinous epithelial neoplasms: A narrative review

Table 1 TNM staging system for appendiceal mucinous epithelial neoplasm, according to the current classification criteria, including definitions of primary tumor (T), regional lymph node involvement (N), and distant metastasis (M), with corresponding stage groupings

TNM staging	Current classification criteria
Primary tumor (T)	Tx	Primary tumor cannot be assessed
	T0	No evidence of primary tumor
	Tis	Carcinoma in situ (intramucosal carcinoma; invasion of the lamina propria or extension into but not through the muscularis mucosae)
	T1	Tumor invades the submucosa
	T2	Tumor invades the muscularis propria
	T3	Tumor invades the subserosa or the mesoappendix
	T4 (T4a, T4b)	Tumor invades the visceral peritoneum, including acellular mucin or mucinous epithelium involving the serosa of the appendix or mesoappendix; tumor invades directly invades adjacent organs or structures
Lymph nodes (N)	NX	Regional lymph nodes cannot be assessed
	N0	No tumor involvement of regional lymph node(s)
	N1 (N1a, N1b, N1c)	Tumor involvement of one to three regional lymph nodes; tumor involvement of one regional lymph node; tumor involvement of two or three regional lymph nodes; no tumor involvement of regional lymph nodes, but there are tumor deposits in the subserosa or mesentery
	N2	Tumor involvement of four or more regional lymph nodes
Distant metastasis (M)	M0	No distant metastasis
	M1	Distant metastasis
	M1c	Metastasis to sites other than peritoneum

Table 2 Differential diagnoses of appendiceal mucinous epithelial neoplasm

Diagnosis	Typical location/origin	Key imaging features (US/CT/MRI)	Helpful differential clues
Appendiceal mucocele/appendiceal mucinous epithelial neoplasm	Appendix, contiguous with cecum	Well-circumscribed ovoid or pyriform cystic mass; “onion-skin” appearance on US; low-attenuation content on CT; T2 hyperintense on MRI; mural or curvilinear calcifications; possible wall thickening or mural nodules	Continuity with cecal pole; separation from right ovary; calcified wall highly suggestive
Acute appendicitis	Appendix	Dilated appendix (> 6 mm), wall thickening and hyperenhancement, peri-appendiceal fat stranding, possible appendicolith	Lack of marked cystic dilatation; absence of mural calcifications or layered mucin
Inflamed mucocele + appendicitis	Appendix	Cystic appendiceal dilatation (> 13 mm), mural calcifications, associated inflammatory changes	Combination of mucocele features and acute inflammation; critical for surgical planning
Peri-appendiceal abscess	Peri-appendiceal region	Irregular fluid collection with thick enhancing walls, gas bubbles, surrounding inflammatory fat stranding	Ill-defined margins; lack of smooth wall and calcifications
Right tubo-ovarian abscess	Adnexa	Complex multiloculated cystic mass; thick enhancing walls; restricted diffusion; surrounding inflammatory changes	Tubal configuration; associated pelvic inflammatory disease; normal appendix
Hydrosalpinx	Fallopian tube	Tubular, elongated cystic structure; incomplete septa; “waist sign”; T2 hyperintense	Tubular morphology; separate from cecum and appendix
Ovarian mucinous tumor	Ovary	Large multilocular cystic mass; variable signal intensity (“stained glass” appearance on MRI); enhancing septa or solid components	Ovarian origin; absence of cecal continuity; more common than appendiceal mucinous epithelial neoplasm
Ovarian serous tumor	Ovary	Unilocular or multilocular cyst; papillary projections; solid enhancing components	Papillary excrescences; ascites without scalloping
Peritoneal inclusion cyst	Peritoneal spaces	Multiloculated fluid collection conforming to peritoneal recesses; ovary trapped within cyst	History of surgery/inflammation; ovary seen inside lesion
Lymphocele	Retroperitoneal or pelvic	Thin-walled homogeneous fluid collection; no enhancement; no calcifications	Post-surgical context; stable over time
Enteric duplication cyst	Adjacent to bowel	Well-defined cystic lesion; double-wall sign; no enhancement	Pediatric or incidental finding; fixed bowel relationship
Cecal carcinoma with appendiceal dilatation	Cecum	Mass-like or circumferential cecal wall thickening; secondary appendiceal dilatation	Primary cecal lesion; solid enhancing mass
Pseudomyxoma peritonei	Peritoneal cavity	Low-attenuation mucinous ascites; loculated collections; scalloping of solid organ surfaces; omental caking; peritoneal implants	Scalloping and relative small bowel sparing are highly characteristic
Lymphoid hyperplasia/inverted appendix	Cecal lumen	Well-circumscribed intraluminal lesion; cystic appearance	No mural calcifications; normal appendix externally

MRI: Magnetic resonance imaging; CT: Computed tomography; US: Ultrasound.